The present invention relates to viscous fluid couplings, and more particularly, to such couplings in which the viscous fluid comprises a magneto-rheological fluid.
Viscous fluid couplings have been in commercial use for many years. A typical use is to drive the radiator cooling fan of a vehicle engine, wherein the engine is of the "north-south" type, i.e., where the axis of the engine is oriented in the direction of the axis of the vehicle. Such couplings are also commonly referred to as "viscous fan drives", and although the present invention is not limited to use as a fan drive, the invention is especially advantageous when used as a fan drive, and will be described in connection therewith.
In typical prior art viscous couplings, especially those used as fan drives, the coupling includes both an operating chamber, where viscous fluid shear occurs, and a reservoir chamber, in which viscous fluid is stored when it is not in the operating chamber. In the prior art coupling, the relationship between output speed and input speed is determined by the extent to which the operating chamber is filled with fluid. In most fan drive applications, the amount of filling of the operating chamber is determined by a valving mechanism, which controls the position of a valve member in response to a sensed temperature condition, such as local, ambient air temperature.
There is an increasing demand for viscous fan drives in which the ratio of output speed to input speed is determined by a remotely sensed temperature condition, such as the "top tank" temperature in the radiator. In such a cooling system, it is desirable to sense the remote temperature, generate a corresponding electrical signal, and transmit that signal to the fan drive, where the signal is used to move a valve member, or somehow vary the transmission of torque from the input coupling to the output coupling.
U.S. Pat. Nos. 4,898,266 and 4,898,267, both of which are assigned to the assignee of the present invention, and incorporated herein by reference, teach viscous fluid couplings in which torque transmission is varied by varying an electrostatic field impressed upon a shear chamber containing an electroviscous fluid. Unfortunately, such electroviscous fluids have not yet been developed to the point where such devices are commercially operable and feasible.
U.S. Pat. No 4,920,929 discloses another general type of viscous coupling in which the ratio of output to input varies in response to variations in an electrical input signal. In the '929 reference, there is a very general disclosure of a coupling in which either an electric or magnetic field is impressed across the viscous chamber, which contains either electro-rheological fluid or magneto-rheological fluid, respectively. However, the device of the '929 reference is shown only schematically, and does not deal with important product issues, such as minimizing the non-working air gaps between the various elements which contain the lines of flux of the magnetic field, or providing a device which can have sufficient magnetic field strength, while having sufficient heat transfer capability to avoid overheating and cooking the fluid.